1. Field of the Invention
The present invention relates to a rotor for an eddy current retarder used as an auxiliary braking system of a large vehicle such as a bus and a truck.
2. Description of the Related Art
A large vehicle such as a bus and a truck is equipped with an auxiliary braking system, in addition to a foot braking system which is a main braking system. As the auxiliary braking system, there is, in addition to an exhaust braking system, an eddy current retarder (hereinafter referred to as "retarder") which is capable of steadily reducing speed when the vehicle is descending a long hill and which is effective for preventing damage of burning the foot braking system.
One example of the eddy current retarder will be explained with reference to FIG. 10. A typical retarder comprises a fixed portion 11 including a plurality of magnets 15 provided around a supporting ring 16; and a rotor 12 comprising a rotary shaft 13, a drum 14 connected to the rotary shaft 13, and cooling fins 17 provided around an outer surface of the drum 14. The rotary shaft 13 of the rotor 12 is connected to a propeller shaft of a vehicle, and the drum 14 rotates around the fixed portion 11 together with the rotation of the propeller shaft. The fixed portion 11 is fixed to the vehicle through a mounting plate. The magnets 15 provided around the outer surface of the supporting ring 16 are electromagnets or permanent magnets, and these magnets are disposed in a staggered manner such that their polarities are alternately different from one another.
In the eddy current retarder of such a structure, when the steel drum 14 rotates around the fixed portion which generates a magnetic field, an eddy current is generated in the drum. This eddy current is converted into heat energy, and the drum is heated. That is, kinetic energy (rotational energy) of the vehicle is converted into heat energy, thereby exerting the braking force on the vehicle.
In such a retarder, the kinetic energy of the vehicle is converted into heat energy through the rotor to exert the braking force. Therefore, as frequency of use of the retarder increases, the temperature of the rotor, especially of the drum and the cooling fins rises. If the temperature of the rotor rises, since braking efficiency of the retarder is lowered and the mechanical strength is also lowered, the retarder is liable to be damaged. Therefore, countermeasures are taken against temperature rise of the rotor by providing a large number of cooling fins around the outer surface of the drum to enhance the cooling ability.
The cooling fins (hereinafter referred to as "fins") are usually disposed around the drum in parallel to its center axis in its circumferential direction at equal distances from one another. When the rotor having the parallel plate type fins rotates, air around the rotor is drawn from both ends of the fins toward the center portion, and forcibly pushed outward from the center portion. The fins are cooled by this air flow and thereby, the drum of the rotor is also cooled. As a type of the cooling fins, in addition to the parallel plate type fins which are parallel to the rotary shaft, there are inclined plate type cooling fins which are inclined with respect to the rotary shaft, and V-shaped plate type cooling fins which are bent in the rotational direction.
In the case of a rotor having the parallel plate type fins or the V-shaped plate type fins among the conventional fin type, since the rotational resistance of the rotor is great, energy loss at the time of normal driving of the vehicle requiring no braking operation is great. Therefore, there are problems that the driving force of the vehicle is lowered and fuel economy is lowered, and there are drawbacks that wind noise is great and noise is great. These problems and drawbacks are caused by the fact that streams of air drawn from both ends of the fins collide with each other at the center portion of the fin and then, the air is sent out and thus, the air does not flow smoothly.
Therefore, for the V-shaped cooling fins, Japanese Patent Application Laid-open No. 7-329740 proposes a method for improving the air flow. This publication discloses a rotor having cooling fins in which a center portion of the V-shaped fins, i.e., a bottom portion of the V-shape is notched to form a slit through which air can pass. It is described that since the streams of air introduced from both sides of the V-shaped fins are discharged rearward of the rotational direction from the slit along the rotor surface, the air flows smoothly, and rotational resistance generated by the air collision can be reduced.
However, in the case of the V-shaped cooling fins having the slit, since the streams of air introduced from the both sides of the fins still collide with each other, a portion of the collided air is discharged outward. Therefore, a remarkable reduction of the rotational resistance can not be expected. Further, the drum and fins of the rotor are heated to high temperature as described above, there is a problem that they are deformed due to thermal stress. In order to prevent the deformation, the fins are usually required to function as reinforcing members. However, if a slit is provided in the central portion of the V-shaped, the drum to which the slit is fixed is varied in strength in the axial direction of rotor depending upon positions of the drum. In such a case, deformation is prone to be concentrated on a portion of the drum where the strengths are different. If the deformation is caused repeatedly, there is a problem that this generates cracks.
Further, in the case of the conventional inclined plate type fins, since the surface areas of the fins can be increased, there are merits that it is easy to enhance the cooling ability and to make the air flow constant, the structure is simple, and it is easy to provide the rotor with the function as the reinforcement for the supporting ring.
In the case of the inclined plate type fins, as disclosed in Japanese Utility Model Application-laid open No. 5-33679 for example, if optimal condition is selected, there is a possibility that the air flow between the fins can be made constant (see FIG. 11). However, if the fin angle is too small, since the length of the fin (length of the fin in the longitudinal direction) becomes long, resistance to air flow between the fins is great, and the air flows hard. Further, since the temperature of air flowing between the fins rises, there is a problem that cooling ability is reduced at a rear portion of the fin. On the other hand, if the fin angle is excessively great, it is difficult to smoothly suck the air toward the fins, and since the length of the fin is short, the surface are as of the fins enough for cooling can not be secured. If the distance between the fins is reduced to compensate the surface areas, since the resistance to air flowing between the fins is increased, there are problems that the velocity of flow of air is reduced and the cooling ability is deteriorated.
As a method for overcoming the drawbacks of the inclined plate type fins, Japanese Patent Application Laid-open No. 6-253527 discloses a rotor having fins comprising a short inclined portion for taking in air, and a main portion which are parallel to the rotational direction as shown in FIG. 12. It is described that according to this rotor, it is possible to make the direction of air flow constant, and to restrain the rotational resistance caused by collision of air at a low level.
However, with the fins of this shape, since the channel of air between the fins becomes long, the temperature of air is high at a rear portion of the fin, and the cooling ability is lowered. Further, since the fins are divided, there are problems that the reinforcing effect of the fins is small and thus, the deformation of the drum can not be prevented, and the machining cost is high.
In the case of a currently used rotor, the shape of a fin is determined mainly in view of easiness of manufacture. Therefore, optimal values of the height of a fin, the distance between fins, fin angle with respect to the rotational direction of the rotor are not elucidated. That is, the real picture is that the shape of fin having the most excellent cooling ability is not grasped.